Sends notifications at lease events

Climate now uses oslo.notify to send notifications when
CRUD lease operations are executed and when relevant lease
events happen (start_lease, before_end_lease, end_lease).
This implementation also adds one new event type called
before_end_lease to be sent when a lease is close to be ended.
Moreover, a new oslo.notify wrapper class was also added.

Change-Id: I861540e5ec5d1309ded2de56b2135bae5b59dad9
Implements: blueprint notifications